A representation is that character of a thing by virtue of which, for the production of a certain mental effect, it may stand in place of another thing. The thing having this character I term a representamen, the mental effect, or thought, its interpretant, the thing for which it stands, its object.

Source:

Peirce, C. S. (1895-6 [c.]). That Categorical and Hypothetical Propositions are one in essence, with some connected matters. MS [R] 787.